SPLITTING THE BLOCK WITH PLUGS AND FEATHERS
To divide a large stone into two or more smaller pieces, the most economical way is to split it. This is the primary means by which marble is quarried and the blocks are divided before sawing.
Plugs and feathers come in several sizes: in a quarry they might use sets to fit in a 1” diameter hole for splitting blocks weighing tens of tons. In the sculptor’s studio the blocks will usually be a fraction of that size, and the plugs and feathers will fit into 3/8” to 3/4” holes. Unless a hammer-drill is available to bore large diameter holes relatively quickly, the smaller sizes will work best.
The holes are drilled slightly deeper than the length of the plugs to be used. The holes are drilled in an evenly spaced row along the line the stone is to be split. Then, sets of wedge-shaped plugs and truncated cone-shaped feathers are fitted into the holes. The series of plugs and feathers is then gradually and systematically driven in tight. This creates a stress fracture and splits the stone along that line of holes.
The number of holes drilled and the distance between the holes will determine the accuracy of the split-fracture. If too few holes are drilled, or if some plugs are driven in tighter than others are, the fracture may be very irregular and even miss some of the holes altogether.
CUTTING STONE
The most common method currently used by stone sculptors to cut stone is to use a continuous-rim dry-cut diamond saw blade mounted on a large right-angle grinder. The diamond blades are made for cutting marble and softer stones. There are saw blades made for cutting harder stones like granite. Do not use the diamond blades for cutting anything but marble and alabaster or they will be ruined. Do not cut sandstone! Diamond blades are primarily for the use of those people working in marble on a larger scale, and rarely will be needed by persons carving alabaster.
Another, less expensive means of cutting the stone is to use a silicon-carbide abrasive disc. The silicon carbide cuts somewhat slower than diamond, and creates more heat generated by friction. This means that the edges tend to spall and chip. The abrasive blades also wear rather rapidly, so the depth-of-cut is always getting smaller.
WORKING SHARP EDGES
If the design requires hard or sharp edges, you will have to treat those edges as fragile high points while carving. Wherever two surfaces meet at an angle, use the flat chisels (or bullnose chisel for concave curves) to cut the edge. After the edge has been cut, then you can go back in and remove the bulk of the waste with the point or the toothed chisels. If the edge is in a place where you cannot chisel, then the diamond saw blade might be used to cut fairly precise edges. The diamond blade is especially useful when cutting sharp edges on the harder stones.
WORKING HOLES OR CONCAVE AREAS
If you want to make a hole or carve a deep concave area, consider the edges of the hole as high points and work the surface away in layers. Don't try to dig straight in like you would with a shovel in dirt, if you do you'll soon split the stone in two. If you bore a hole with a drill first, you can easily carve it larger and modify the shape with chisels and abrasives.
SMOOTHING THE SURFACE
After the carving work is complete on pieces which are to be finished smooth or polished, the next step in the process is to remove the chisel marks and to contour the surface. Depending upon the hardness of the stone this can be done in several ways.
On soft stones like alabaster rasps and files are used. Some types of marble are soft enough to allow the use of rasps and files as well. For the harder varieties of carving stone, rubbing stones are used to abrade the surface. Rasps, files, and rubbing stones all serve to give uniformity to the surface contours. The uniformity derives from the systematic wearing-down of all the high points of the surface so that the removal of the chisel marks leaves contour changes that are not abrupt or discontinuous.
RASPS AND FILES
Most rasps and files are made of high-carbon tool steel and they have teeth that are designed to cut when pushed in one direction (away from the handled end). Rasps have a series of individual pointed teeth that are made for rapid stock removal. File teeth are a regular series of long cutting edges that give a much smoother cut than the rasp. Rasps and files are made in a variety of shapes for a variety of contours. A half-round rasp is a basic shape, although all of the various shapes will be of use to the carver. The odd curves and angles of riffler rasps and riffler files are especially useful while working hard-to-reach areas of the carving. The types shaped like short knife blades and the types shaped like small spoons will be the ones most often used.
There are also tools called ‘rasps’ or ‘files’ that are actually abrasive tools without actual teeth. They have diamond or tungsten carbide grit fused to the working surfaces. This means that they will work with strokes in any direction. Because these grits are much harder than steel teeth, these tools are especially useful when working the harder varieties of stone.
RUBBING STONES
Sandstone is composed of hard particles cemented together within a softer matrix. The particles in most sandstones are harder than most of the stones used for carving. Sandstone, rubbed over a piece of marble, will work like a piece of sandpaper and smooth the marble. Until the end of this last century, natural rubbing stones, such as sandstones, were used to smooth the surfaces of stone sculptures. Rubbing stones used now are blocks of the synthetic substance silicon carbide (SiC). Silicon carbide is one of the hardest man-made materials besides diamond, and it is used as an abrasive throughout the stone-working industry. Long lasting diamond-coated flexible rubbing blocks are also used in the trade, but they are substantially more expensive than silicon carbide rubbing blocks.
Like sandpaper, rubbing stones (sometimes called blocks or bricks) come in different grits and should be used sequentially from coarse to fine. The silicon carbide rubbing stones can be broken and shaped so as to fit particular contours on the surface of the sculpture. Although rubbing stones can be used dry, they work better if used with water.
SANDPAPER
After the contours are shaped with the rasps, files, and rubbing stones, the surface can be further smoothed with 'sandpaper'. The silicon carbide sandpaper used for stone is waterproof and should be used with water. The water helps to wash away the sanding dust and any grit that comes loose from the paper.
Each grit of sandpaper is used to remove the scratches left by the previous grit. The full range of grits should be used if possible, and the stone should be thoroughly rinsed between grits. When grit 600 has been finished, all the visible scratches should be gone.
Rinse the stone with clean water and let it dry in the sun. After it is dry, check the surface carefully for any stray scratches remaining. If there are any, go back over those areas with the appropriate grits. When the dried surface looks completely scratch-free the surface is ready for the final polishing.
